1. Field of the Invention
This invention relates to D/A converters (DACs) comprising an MOS integrated circuit chip having a number of transistor current sources the outputs of which are switchable between an output line and a reference line such as analog ground. More particularly, this invention relates to such converters having means to reduce the amount of current switched to the reference line so as to reduce the overall power consumption of the DAC.
2. Background of the Invention
Many modern D/A converters employ an array of MOS transistor current sources which are switchable by the digital input signal either to an output line or to ground. When the current flows to ground, it performs no useful purpose yet can excessively heat the IC chip and may also require undesirably large size conductors for carrying the supply current into the chip.
One way to reduce this unwanted current flow is to simply open-circuit the current source when the output line switch is turned off. This approach is not satisfactory however because it produces an unacceptable voltage overshoot at the analog output when the output-line switch subsequently is turned back on. This is because there is a fast voltage transition at the MOS current source drain at switch turn-on, and this voltage transition is coupled through to the output line to create a voltage overshoot.